Central to the operation of a radio communications device is receiving an electromagnetic radio frequency (RF) signal and decoding the data embedded in it. The RF signal can, however, be seriously degraded during signal transmission. For instance, the signal strength is attenuated as it travels through air, with the amount of attenuation generally increasing with the distance traveled, before consideration of multi-path effects. The effect of signal loss is exacerbated by the noise and interference introduced into the signal along the communication path. The noise and interference can be particularly problematic when introduced at the receiver where the communicated signal is weakest, thereby resulting in low signal-to-noise-and-interference ratio (SNIR) conditions and making signal decoding difficult. In severe cases with low SNIR levels, the system can be prone to making decoding errors.
The problem of electromagnetic interference (EMI) is of interest because it can be considerably stronger than other noise sources and can thus be the dominant system impairment, or in other words, the active constraint limiting system performance. Such interference can arise from a variety of sources. One illustrative EMI source is other radio communications devices. As other radio devices transmit RF signals amongst themselves, their RF signals can also be received by other unintended devices. To these unintended devices, those RF signals can be nuisance interference. When such unintended signals are strong enough, they can overwhelm the desired received communications signal.
A second illustrative source of EMI is the communication of data along buses or traces, e.g. the sending of electrical signals along conductive or metal paths embedded in a dielectric, plastic, or other molding material. When a signal is transmitted at a high speed or high baud rate, a conductive trace or bus can active like antenna and emit electromagnetic radiation. This radiation may then be absorbed as EMI by an antenna or another bus or trace in the receive path of a communications device.
The problem of EMI has recently been garnering attention because of its root at the conflict between communications performance and prevalence of use. The use of communications devices and services is experiencing a growth explosion in services such as WiFi (IEEE 802.11), cognitive radio (IEEE 802.22), mobile phones (e.g. GSM, EDGE, CDMA, W-CDMA, WiMAX, LTE), global positioning service (GPS), Bluetooth, and mobile video (e.g. DVB, DMB, MediaFLO). The concurrent use of two or more of these services in close proximity, however, can cause one service to interfere with another. In some instances, multiple services can be supported on the same device where the physical distance between the radio antennae is very short (on the order of centimeters apart) or even share the same antenna such as in a GPS, mobile video, and WiFi enabled 3G mobile phone. On such a device, if a first service such as WiFi or WCDMA is transmitting while a second service such as GPS or DVB-H is receiving, then it can easily be the case where the transmitting first service signal is in excess of 10,000 times stronger than the receiving second service signal thus corrupting the receiving second service with overwhelming interference. A number of other illustrative interference scenarios in commercial applications, though not an exhaustive listing, can be found in the presentation “Performance Analysis and Design Considerations for Multi-Radio Platforms” by Waltho et al. delivered at the 2006 Intel Developers Forum.
The technological advancement of communications devices is being impeded by increased interference not only because of the number of radios used in a confined area but also because of the denser and faster signal routing in newer communications devices. As previously noted, a high-speed signal path can emit EMI that can be absorbed by a nearby radio receiver. Thus, even on devices where the radios are carefully controlled, e.g. there is no concurrent operation of multiple radio services, the operation of other non-radio aspects of the device during radio communication may pose a problem. For example, in a camera-enabled mobile phone, a ribbon cable bus between a processor and camera module may emit enough EMI to disrupt phone reception resulting in the interruption of an ongoing call or the missing of an incoming call.
EMI can impede the integration of wireless radio services and other technologies involving high-speed signal paths into a single, small form factor device. Consequently, there have been many efforts to address the problem of EMI in a communications receiver. Some of the conventional art has proposed the use of high quality filters to suppress out-of-band interferers (i.e., EMI whose frequency spectral coverage is disjoint from the spectrum utilized by the primary radio communications receiver). Such filters include surface acoustic wave (SAW), bulk acoustic wave (BAW), and film bulk acoustic wave resonator (FBAR) filters. While such filters offer very high suppression of out-of-band interferers, their use has at least two significant drawbacks. First, these high-quality filters commonly have undesirably high component costs, in terms of dollar pricing. Second, these filters often come as discrete components which are not integrated into other existing components on a communications receiver. Thus, their inclusion results in an increase in area utilization. Furthermore, the EMI problem is severe enough in many contexts that multiple filters have to be cascaded to provide adequate isolation, thus multiplying the price and area costs associated with that solution. Many consumer products, such as mobile phones, GPS receivers, and portable digital assistants, are highly sensitive to pricing and physical size and would be more marketable absent increases in these characteristics.
Another major group of efforts to reduce the impact of EMI are based on the principle of interference cancellation or suppression. The general principle of interference cancellation in the prior art is illustrated in FIG. 1 where a victim receiver 120, as part of a first radio communications system, is operating concurrently with an aggressing transmitter 110 as part of a second system, and the signal from the aggressing transmitter 110 couples into the receive signal path via the coupled signal 150. An active cancellation unit 130 samples the output from the aggressing transmitter 110 and applies a transformation (e.g. attenuation, delay, and/or phase shift), to mimic the transformation that occurs in the coupled signal 150. The victim receiver 120 then subtracts the output from the active cancellation unit 130 from the corrupted received signal. The resulting signal then has the interference removed to the degree that the output of the active cancellation unit 130 models the coupled signal 150.
The prior art in interference cancellation suffers from several major shortcomings. One significant drawback is the fundamental requirement that the active cancellation unit 130 senses (or equivalently taps or splits) the output of the aggressing transmitter 110 as proposed in U.S. Pat. No. 6,539,204 by Marsh and Sutton, U.S. Pat. No. 6,915,112 by Sutton and Soledade, U.S. Pat. No. 6,745,018 by Zehavi et al., U.S. Pat. No. 7,123,676 by Gebara et al., and U.S. Pat. No. 7,522,883 by Gebara et al. The model of the coupled interference signal 150 is generated by adjusting the amplitude, phase, and/or delay of the transmitter 110 output. Without such a sensing of the aggressing transmitter 110, the active cancellation unit 130 will not produce a meaningful model of the coupled interference signal 150, and consequently, will not be able to reduce the interference. However, such a sensing is highly undesirable and in many cases impossible. Indeed, it is undesirable to sense the aggressing transmitter 110 output because doing so can distort the signal transmitted from the antenna 140. In many applications, the signal launched from the antenna 140 must adhere to strict regulatory requirements such as a spectral mask, and systems designers take great steps to do so. However, the addition of a sensor on the transmit path could significantly alter the properties of the transmitted signal causing it to fail the requirements and thus necessitate a major, sometimes impractically laborious, redesign of the communications system. Thus, the addition of the sensor near transmitter 110 can preclude the active cancellation unit 130 from being added to an existing communications device design to alleviate a previously unanticipated EMI problem. Furthermore, in many instances, the aggressing transmitter 110 is either unknown or not on the same device as the victim receiver 120 thus making sensing impossible. An example of an unknown aggressing transmitter could be a high-speed bus or signal trace that emits EMI. In modern communications systems, there are myriad such buses that can cause problematic EMI and identification of the one or more offenders is impractical. Even if the offending EMI sources could be identified, it would likely be impractical to sense them because of routing complexities from the sensing point to the victim receiver.
While not requiring explicit sensing of the aggressing transmitter, some methods in the prior art still require prior coordination between the aggressing transmitter 110 and victim receiver 120. U.S. Pat. No. 7,443,829 by Rizvi et al. for example only cancels uplink interference in a CDMA based network. Such an approach is limited because it can only cancel interference from other CDMA signals, i.e. it does not mitigate interference from non-CDMA sources. Furthermore, among CDMA interference sources, that approach only cancels those that are received at the same basestation as the victim receiver. Such a cancellation technique is very limited in the types of communications systems to which it can benefit.
Another problem with methods for interference cancellation in the prior art pertains to those that require the aggressing transmitter 110 to have prior knowledge of the carrier frequency of the victim receive signal. Such an example of this is in U.S. Pat. No. 7,058,368 by Nicholls and Roussel where they cancel the interference at the source of the aggressing transmitter 110 by suppressing transmit energy over a specified bandwidth corresponding to the spectrum used by a victim receiver 120. In order to do this however, the spectrum band used by the victim receiver 120 must be known to the aggressing transmitter 110 in order for the latter to suppress energy in the proper band. Requiring such knowledge is undesirable because in many situations aggressor-victim pairs cannot be known ahead of time and are constantly changing. In particular, a potential aggressor 110 may not be able to know what victim radios 120 it will be aggressing. In fact, the aggressor 110 may not even be an explicit radio transmitter but may instead be a high-speed signal path or processor.
An additional problem with interference cancellation methods in the prior art relates to those that require modification of the aggressing transmit signal. For example, U.S. Pat. No. 6,539,204 by Marsh and Sutton, U.S. Pat. No. 6,745,018 by Zehavi et al., U.S. Pat. No. 7,123,676 by Gebara et al., and U.S. Pat. No. 7,058,368 by Nicholls and Roussel propose the injection of a pilot or reference tone into the signal transmitted in or near the aggressing transmitter 110. The presence of the pilot signal is then used at the victim receiver 120 to identify the coupled aggressor signal 150 and guide the control of the cancellation unit 130 to maximize the degree of cancellation. The modification of the aggressing transmit signal however has several major drawbacks. First, modifying the aggressing transmit signal after the output of an aggressing transmitter 110 is likely to disturb the signal integrity of the transmitted signal. Such degradation may come in the form of loss of valuable signal power or in terms of signal distortion. These degradations can be severe enough to cause the signal to violate required signal criteria such as those imposed by spectral masks. If, however, the modification to the aggressing transmit signal is incorporated in the aggressing transmitter itself, then such an approach requires customized transmitters where the particular interference cancellation technique, aggressing transmitter, and victim receiver are all designed in a coordinated fashion. Such high degrees of customization are not practical for large scale adoption and the variety of interference pairings that can be encountered. Another drawback of modifying the transmitted aggressor signal is that doing so requires identification of and physical access to the interference source. As previously noted, in many contexts, the aggressing source is not known or not on the same device as the victim receiver and thus not available for modification.
Besides injecting a reference pilot tone into the transmitted aggressor signal prior to transmission over an antenna, a reference tone can be injected after reception by the victim receiver. Such an approach is promoted in U.S. Pat. No. 7,123,676 by Gebara et al. and U.S. Pat. No. 7,058,368 by Nicholls and Roussel. However, in order for such an approach to be effective, the victim receiver must use a pilot tone whose frequency is congruous with the frequency of the transmitted aggressor. In other words, the victim receiver must know the operating frequency of the aggressing transmitter in order to accurately set the tone of the reference pilot to provide good guidance for cancellation. Unfortunately, in many cases, the identity of the aggressing transmitter, and especially its operating frequency, is not known to the victim receiver. Furthermore, as known to those skilled in the art, the act of injecting a tone, or any other signal, into the received victim path can degrade the signal fidelity of the already weak and vulnerable received victim signal.
Avoiding the use of a pilot tone, an alternate approach for optimizing the parameters in an interference cancellation unit is to have the victim receiver 120 feed a signal fidelity measure back to the cancellation unit 130 to guide the adjustment of the cancellation parameters. For example, U.S. Pat. No. 6,915,112 by Sutton and Soledade and U.S. Pat. No. 7,123,676 by Gebara et al. proposed the minimization of a victim receiver's Received Signal Strength Indicator (RSSI) measure to guide the control of the cancellation unit. Use of such a signal, external to the cancellation unit, has several drawbacks. First, such an approach requires the delivery of the external fidelity measure from the receiver 120 to the cancellation unit 130. Oftentimes, such a fidelity measure is not available externally from the receiver 120 for such use, and even in cases where it is available, making use of the signal can be burdensome due to the extra routing or processing resources needed. Second, indirect measures such as received power or RSSI are not always indicative of interference power or lack thereof. For example, the amount of residual interference (after cancellation) may increase while the received victim power degrades due to a fading communications channel. In this case, the net result can be a decrease of total received signal power or RSSI in which case the cancellation unit will be misguided into believing that the amount of interference has been reduced because the cancellation unit has no means of determining the individual contributions of the received victim signal and the interference signal to the total signal power or RSSI.
Besides the inappropriateness of general fidelity measures for the control of an interference cancellation unit, many methods in the prior art propose the less than desirable optimization technique of trial-and-error coordinate descent optimization. For example, U.S. Pat. No. 6,915,112 by Sutton and Soledade and U.S. Pat. No. 7,123,676 by Gebara et al. propose to take a first fidelity measurement under a first set of cancellation parameters followed by a second fidelity measurement under a second set of parameters. Then, the set yielding the better fidelity measure is selected as the better set. This trial-and-error approach can be repeated until some stopping criterion is met, or it can be continued indefinitely. The drawback of trial-and-error coordinate descent minimization is that the cancellation unit is always forced to test a parameter set which will have an unknown and perhaps worse performance. It would be strongly preferred to use an optimization technique, such as a gradient descent-based technique, where from a single parameter set, one is able to determine how the parameters should be modified to improve fidelity, i.e. without the testing of a second set of parameters. It is well known to those skilled in the art of controls theory that gradient descent-like techniques outperform trial-and-error coordinate descent both in terms of resulting signal quality and the speed in which the optimum system parameters are achieved.
Some approaches in the prior art proposed to cancel signal interference on the baseband signal, i.e. after the signal has been modulated from its RF carrier down to a low frequency or DC carrier and quantized by an analog-to-digital converter (ADC). Such an approach is proposed in U.S. Pat. No. 6,539,204 by Marsh and Sutton, and U.S. Pat. No. 6,745,018 by Zehavi et al. because of the sophisticated digital signal processing techniques that can be used to remove interference. However, removing interference at the baseband level has several major drawbacks. Such an approach places extreme performance requirements on the front-end circuitry components of the receiver. In particular, when a weak received victim is in the presence of a strong interferer, the down-conversion mixer in the receiver will oftentimes further distort the signal as nonlinearities are introduced due to practical limitations on circuit linearity. Furthermore, because the interference signal is so much larger than the received victim signal, most of the dynamic range of the ADC is wasted to accommodate the large interferer. Thus, either very expensive high performance front-end circuit components are required or additional signal degradation is introduced. Thus, there is a need in the art for interference cancellation techniques that cancel the interference early in the front-end of the receiver before the down-conversion modulation or analog-to-digital conversion.
Accordingly, there is a need in the industry for systems, methods, and apparatuses for reducing interference at the front-end of a communications device to address one or more of the above-described deficiencies or yet other deficiencies.